Priority to German Patent Application No. 101 52 158.8, filed Oct. 25, 2001 and hereby incorporated by reference herein, is claimed.
The present invention relates to a method for controlling the ink feed of a machine which processes printing substrates and features at least one inking unit and a computer and to a device which is suitable for carrying out the method.
In the production of printed matter, it is important, above all, that the products leaving a printing press correspond as far as possible to an original copy provided by a customer. This results in complex adjustment procedures, in particular, in the case of sheet-fed printing presses, because, compared to web-fed rotary printing presses, the jobs change much more frequently here. The settings of the printing press have to be changed for each job change, which is very time-consuming. In addition to changing the printing plates on the individual printing units of the printing press, these settings also include those the for the inking unit of each printing unit, especially when the inks in the inking units have to be changed. In this context, the adjustment of the inking unit and thereby of the ink feed depends on many parameters, including the printing speed of the printing press as well as environmental factors such as air humidity and temperature in addition to properties of the printing substrates and the printing inks.
To avoid unnecessary spoilage, state-of-the-art printing presses are calibrated to specific printing inks and specific paper so that spoilage can be reduced using these specific consumables. However, this results in only very limited success because the printing ink properties are subject to relatively large variations, resulting in considerable spoilage in spite of the calibration. For this reason, many printing presses operate with measuring systems which measure the finished, printed sheets or parts of the sheets opto-electronically, mostly on the basis of spectra, the measurements being subsequently compared to an original copy which is measured in the same manner. The differences between the original copy and the printed product determined in this comparison are then used to appropriately adjust the ink feed of the inking units until, in accordance with the requirements, the original copy and the printed product no longer differ. In this case, an OK sheet exists and the print run can start.
A method of that kind is known from European Patent Application No. EP 0 585 740 A1, where screen tints of individual printing colors or of the whole print are photoelectrically scanned and the reflectance values obtained in the process are converted to a characteristic curve. The characteristic curve determined in this manner is adjusted to a predetermined reference characteristic curve by influencing the printing process accordingly. The adjustment of the actual characteristic curve to the predetermined reference characteristic curve is accomplished in that a parameter of the actual characteristic curve is varied and in that, using a performance index, that actual characteristic curve of the resulting actual characteristic curves is selected which leads to a particularly good match with the reference characteristic curve. Hence, this is a closed-loop control circuit which measures reflectance values of a printed product and subsequently changes the characteristic curves stored in the printing press accordingly. However, a closed-loop control circuit of that kind can only react because consumption parameters such as ink and paper cannot be taken into account by the control, as a result of which considerable spoilage is produced until the OK sheet is achieved.
An object of the present invention is to provide a method and a device for carrying out the method which are capable, in particular, of taking into account the properties of consumables and of adapting the ink feed according to the properties already before the start of printing.
The present invention provided a method for controlling the ink feed in a machine which processes printing substrates (12) and features at least one inking unit, wherein a computer knows at least the physical properties of printing ink and/or printing substrates (12) as data; the stored data is read into an ink control model which is stored in the computer; and the optimum settings with regard to the ink feed are made on the basis of this ink control model before the start of printing or during the printing process. A device for performing the method is also provided.
The method according to the present invention can preferably be implemented on a printing press having a control computer which is able to exchange signals for controlling one or more inking units. However, it is also possible that a separate computer exists which calculates the optimum settings using the ink control model and that the data is transmitted from the computer to a printing press or to manually entered there. The ink control model can also be implemented on a computer which is provided in the printing press in addition to the control computer. Besides, the data can also be calculated in the preliminary stages. The place of calculation does not matter, it is only crucial that the physical properties of the consumables be available to the ink control model on a computer and that the results be subsequently used to control a printing press.
The control computer or separate computer has stored therein the ink control model in which the physical properties of the consumables such as printing ink and printing substrate or paper, as well as ambient parameters such as air humidity and temperature are mathematically correlated. Because of this, the ink control model stored in the computer is able to control the ink feed of the inking unit on the basis of the physical properties of the printing ink or printing substrates and of the ambient parameters in such a manner that as little spoilage as possible is produced and the set-up time during job change-over can be minimized. To this end, the physical properties of the consumables must be known, whether they are provided by the consumable supplier or whether the consumables are measured accordingly and thus the physical properties are known on the basis of the measurements. Since the different properties of the consumables and the ambient parameters are known and can be introduced into the control of the printing press, the ink control functions of the printing press can be optimally adapted, and much less deviation from the expected printing result is to be expected than if the printed copies were measured only during printing, as in the related art, and possibly considerable deviations from the original copies would have to be found. In the related art, it can take a very long time until the quality of the printed products has reached a satisfactory degree, especially when the physical properties of the printing inks or printing substrates strongly deviate from the usual standard. Using the method according to the present invention, these problems can be prevented to the greatest possible extent through the a priori knowledge of the properties of the printing substrates and printing inks and through the ink control model.
In a first embodiment of the present invention, provision is made for the stored inks to be used by the ink control model for optimum ink presetting. Optimum ink presetting is an important aspect for achieving good printing results in a rapid manner and with as little spoilage as possible. Using the ink control model, the ink presetting can be optimally adjusted in such a manner that no complex readjustments are required during the printing process, as in the related art.
Moreover, provision is made for the stored data to be used by the ink control model for speed compensation in case of a change in printing speed. For each printing speed, there is a printing unit setting that leads to optimum printing results. As soon as the printing speed is changed, the ink feed has to be changed accordingly because otherwise the print quality will deteriorate. To achieve as rapid a speed compensation as possible, it is therefore a great advantage that, using the ink control model, which also takes into account the printing speed, it is possible to precalculate the inking unit settings that are required for the printing speed to be attained. Thus, no complex control loop needs to be started to effectively reduce spoilage during speed changes.
A further advantageous embodiment of the present invention is achieved in that the stored data is used by the ink control model to optimize the pre-inking and/or the ink profile removal, for example through ink doctor removal, during a change in print job or after washing the printing unit. When a change in print job is imminent, the printing ink present in the inking units must be removed to an extent that the inking unit settings can be made for the following print job. The ink must also be built up anew subsequent to washing the inking unit. To minimize the set-up time during a change in print job, it is important that the ink be removed only to the extent that is absolutely necessary. Otherwise, an unnecessarily long time for the ink build-up required for the following print job has to taken into account during the subsequent pre-inking. Therefore, it is a great advantage for the ink removal and the pre-inking for the subsequent print job if the corresponding values for the ink removal and the pre-inking can be precalculated using the ink control model. This is important especially when the printing substrate is changed and the inking unit settings can be adapted accordingly.
In one embodiment of the present invention, provision is made for the data on the physical properties of the printing ink to include spectral reflectance values of the printing ink on the printing substrate used for the printing process. The reflectance values of a printing ink also depend, inter alia, on the printing substrate onto which this printing ink is applied, on the dry state of the ink, and on the layer thickness in which it is printed. Thus, the reflectance values of the printing ink can only be optimally taken into account in the ink control model if the reflectance values of the printing ink were measured on the printing substrate that is actually used in the printing press. This can be accomplished by preliminary spectral measurements of the printing ink on the printing substrate used.
Moreover, it is advantageous that the data on the physical properties of the printing ink includes spectral reflectance values of the printing ink on standard printing substrate. If the spectral reflectance values of the printing ink on the printing substrate which is used in the printing press are not known, then it is required to measure the printing ink on the corresponding printing substrate in advance. To avoid such a preliminary measurement, it is also possible to use values which represent the reflectance values of the printing ink on a standard printing substrate. On the basis of the data on the printing substrate which is actually used for the printing process, the spectral reflectance values of the printing ink of the standard printing substrate are then appropriately converted so that the reflectance values of the printing ink become also meaningful for the printing substrate actually used.
In one embodiment of the present invention, provision is made for the data on the physical properties of the printing ink to include spectral reflectance values for at least two layer thicknesses of the printing ink. In this manner, it is possible to calculate the optimum inking unit settings for a desired coloring using the ink control model.
Moreover, provision is made for the data on the physical properties of the printing ink to include rheological properties under standard conditions. The science of rheology deals with the flow and deformation behavior, in particular, of liquid substances. Rheological parameters include, inter alia, the viscosity and tack of the ink. Thus, the rheological properties of a printing ink depend, inter alia, on the viscosity and the ambient temperature. If the rheological properties of the printing ink under standard conditions as, for example, a certain viscosity range at a certain ambient temperature are known, then the rheological properties of the printing ink can also be obtained for changed conditions by conversion using the ink control model. In this manner, the rheological properties of the printing ink can be integrated into the ink feed.
Moreover, it is an advantage if the data on the physical properties of the printing ink includes a maximum dampening agent absorption capacity and/or a maximum dampening agent absorption rate under standard conditions. Besides an ink metering device, there also exists a dampening agent metering system in a normal printing unit of a printing press. In this manner, dampening agent is supplied to the printing ink on the rollers of the inking unit before the printing ink reaches the plate cylinder of a printing unit. The properties of the printing ink can be influenced via the dampening agent so that it is advantageous if the properties of the dampening agent are also taken into account in the ink control model. Here too, it is possible to infer values under changed conditions from values which are measured under standard conditions.
In a further embodiment of the present invention, provision is made for the physical properties of the printing substrate to include as, for example, surface properties and spectral reflectance values of the of the printing substrate used for a print job or a printing substrate classification. In this manner, the properties of the different printing substrates are introduced into the ink control model and can be taken into account in the ink feed. If the properties of a printing substrate used are not available, then it can possibly be sufficient to know at least the printing substrate classification such as glossy coated, matt coated or uncoated.
It is particularly important that further printing parameters such as the printing unit temperature and/or the printing speed and/or the zonal coverage be available to the computer as data. This data is eminently important to be able to ensure optimum ink feed for a print job. The printing speed can easily be obtained from the machine data of the printing press and thus be fed to the computer. For the inking unit temperature, a suitable thermometer has to be provided which serves as a sensor for the computer and supplies it with the corresponding data. The zonal coverage must be entered into the computer prior to the start of the printing process.